The unaware, vaguely idiotic central character provides a deep mine of material for any type of slightly tongue in cheek story-telling, and UNFAITHFUL UNTO DEATH uses the premises in setting up Dr Cyril Peabody from the outset of the novel. Cyril is perpetually disappointed in life. He has been stymied in his career path, forced to take a (in his opinion) menial job as a country GP, married a woman who is only just satisfactory, and generally living a life that he feels has been affected constantly by the wilfulness of others. Obviously he's completely incapable of seeing that he's the problem. He's boorish, prissy and prone to conflation of his own worth. He's basically a tiresome individual.
Writing about these sorts of characters is a tricky undertaking, as balance between time spent with somebody who is absolutely slappable and actual advancement of plot, hopefully to where Dr Cyril gets his comeuppance, has to remain engaging for the reader. Alas UNFAITHFUL UNTO DEATH dwells a lot on the man, which whilst you can see there is humour there, the joke becomes thin quickly. There's even something oddly off-putting about wife Rosamund, who if anybody had a right to some happiness, alas has her own overly annoying quirks without enough of the humour to humanise her.
It has to be noted that humour of this type is a difficult undertaking as a reader's experience is greatly affected by their relationship with the Rosamund and Dr Cyril. Overall, a little more acuity and balance between the ego of Cyril, the passivity of Rosamund and plot advancement would have helped this reader a lot.